1. Field of the Invention
This invention relates generally to the assay of microliter amounts of a fluidic sample using magnetic resonance, and more particularly to nuclear magnetic resonance (NMR) imaging methods and apparatus incorporating magnetoresistive sensors for the conduct of such assays.
2. Background of the Invention
Miniaturized fluid handling devices have recently attracted considerable interest in many areas of science. Such microfluidic chips perform a variety of functions, ranging from analysis of biological macromolecules to catalysis of reactions and sensing of fluids in the liquid or gas phase. Some microfluidic chips also enable assay of combined multiphase reactions (solid-liquid-gas).
Currently, most microfluidic flow measurements rely on optical detection of markers, requiring the injection of tracers and use of transparent devices. These techniques, however, by their very nature result in a degree of sample contamination due to the addition of the marker. Moreover, optical methods may not be applicable for chips or where the fluidic sample is optically opaque. Another drawback of these techniques is their lack of specificity, where one seeks to differentiate between different chemical species in a fluidic sample, or, more generally, where one seeks to track two or more species together, given the limitations of a marker. Suggested approaches which do not require the use of injected markers include the use of superconducting quantum interference devices (SQUIDs) and the use of alkali-vapor atomic magnetometers. However these devices require cryogenics in the case of the former, or heated vapor cells in the case of the latter. For microfluidic measurements, there thus remains a need for a detection system which does not have these drawbacks, is compact and relatively inexpensive.